A typical STT-MRAM (spin transfer torque magnetic random access memory) MTJ (magnetic tunnel junction) device has a pinned reference layer whose magnetization is fixed in certain direction by either intrinsic anisotropy field, or through an exchange coupling field from an adjacent magnetic layer. It also has a switchable free layer, whose magnetization direction can be switched in either of two directions relative to that of the reference layer by an electric current flowing between the reference layer and free layer through an junction layer, typically an oxide of Mg, Al and Ti, or a metallic layer of Cu, Au, or Ag. The different relative angles between free layer and reference layer magnetization directions gives different resistance levels through the MTJ stack. Thus, by switching the free layer magnetization directions with the applied electric current, an STT-MRAM can be switched into high and low resistance states. The low resistance state occurs when the magnetization orientation of the two ferromagnetic layers is substantially parallel and the high resistance state occurs when they are anti-parallel. Therefore, the cells have two stable states that allow the cells to serve as non-volatile memory elements.
Reading the state of the cell is achieved by detecting whether the electrical resistance of the cell is in the high or low state. Writing the cells requires a sufficiently high DC current flowing in the direction through the MTJ stack between the top and bottom metal contacts to induce a spin transfer torque (STT) that orients (switches) the free layer into the desired direction. The amount of current needed to write the cells is higher than the current needed during the read process, so that a read operation does not change the state of the cell.
A study by Wang, et al. on perpendicular MTJ shows that the perpendicular anisotropy of magnetic layers in MgO based MTJ structures can be changed by the voltage applied to the magnetic layers. See Wei-Gang Wang, et al., “Electric-field-assisted switching in magnetic tunnel junctions”, Nature Materials Vol. 11, 64-68 (2012). Wang, et al. used an example MTJ layer structure (which is illustrated in FIG. 1 herein) of bottom magnetic layer 43 of CoFeB (1.3 nm), MgO layer 42 (1.4 nm), and top magnetic layer 41 of CoFeB (1.6 nm). In the test setup as shown a small positive DC electric potential is applied to the MTJ cell to drive electrons into the bottom magnetic layer. When MgO layer 42 is thick enough and resistance across the MgO junction is high enough, the current density through the MgO junction will be low. In this case, the two magnetic layers adjacent to the MgO layer form a capacitor across the MgO layer, which is fundamentally the same as a classic parallel-plate capacitor with the MgO layer as the dielectric between the parallel plates. When a voltage is applied to the MgO junction, electrical charges will accumulate in the two magnetic layers, which is governed by the capacitor equation of Q=C×V, where Q is the net charge, C the capacitance and V the applied voltage. The applied positive voltage as shown in FIG. 1 causes the top magnetic layer to have a positive potential, i.e. electron depletion at the top layer's interface to the MgO layer and results in the flow of electrons toward the positive potential. Wang's graph (reproduced in FIG. 2 herein) shows the coercivity field Hc for the top and bottom layers as a function of the electric field. The perpendicular anisotropy is reflected by the measured coercivity field Hc. With increasing applied voltage and electron depletion at the top layer's interface to the MgO layer, the top magnetic layer shows increased perpendicular anisotropy. For the magnetic layer with positive potential, the positive charges at its interface with MgO are basically vacancies of conductive electrons that are depleted by the applied voltage. The layer that has negative potential (the bottom layer in this example) and, therefore, conductive electron concentration at the layer's interface to MgO, shows decreased perpendicular anisotropy.
The fundamental difference between an MgO junction acting as a capacitor and a standard capacitor is that the electrons at the magnetic layers interface also induce magnetic anisotropy in the magnetic layers. The cited Wang, et al. and Alzate, et al. articles show surface perpendicular anisotropy of CoFeB layers on the sides of the MgO layer is intrinsically due to the broken-symmetry of the interface CoFe lattice of the CoFeB layer facing the MgO layer. See J. G. Alzate, et al., “Voltage-Induced Switching of Nanoscale Magnetic Tunnel Junctions”, IEDM digest, San Francisco, December 2012. FIGS. 4A and 4B illustrate the voltage-induced switching principle describe by Alzate, et al. In a perfectly symmetric and continuous lattice of CoFe, the electron-to-electron spin exchange coupling between the un-paired 3d electrons, which are also the conductive electrons, of Co and Fe atoms, cancel out each other's effect and produce zero anisotropy energy in a symmetric and continuous lattice. However, at the interface of CoFeB layer and MgO layer, the CoFeB layer's interface is actually CoFe facing MgO. MgO breaks the symmetry of the CoFe lattice, such that the 3d electrons of Co and Fe atoms at the interface lose their cancellation-counter-part and produce a net anisotropy energy and an effective anisotropy field perpendicular to the interface plane. Thus, under these conditions the originally soft magnetic CoFeB film can exhibit strong perpendicular anisotropy at the MgO interface and show hard magnetic behavior.
Wang, et al. show that with applied voltage, the magnetic layer subjected to a positive potential, i.e. electron depletion at the layer's interface to MgO, shows increased perpendicular anisotropy. The magnetic layer that experiences a negative potential, i.e. electron concentration at the layer's interface to MgO, shows decreased perpendicular anisotropy. A possible cause of such behavior is that the magnetic layer having increased electrons will have more conductive electrons filling into the 3d-band of the interface CoFe lattice and reduced unpaired 3d-electron population; thus making the broken-symmetry induced surface perpendicular anisotropy weaker and making the magnetic layer magnetically softer. For the magnetic layer having 3d-electrons depleted, electrons will be first depleted from paired 3d-electrons due to Hund's Rules. More 3d-electrons become unpaired, which enhances the surface perpendicular anisotropy and makes the layer harder to switch by external field or STT.
The voltage-induced perpendicular anisotropy effect can be used to switch MTJ by combination with spin transfer torque effect. Using an in-plane MTJ, for example, by applying a positive voltage across MTJ, which depletes electron from the free layer/MgO interface, a strong perpendicular anisotropy on the free layer will be induced, and therefore, reduce its coercivity and make it easier to switch. While the coercivity is reduced, the free layer magnetization can be set/switched with a smaller magnetic field than is needed under static conditions. It has been shown that this small magnetic field can be replaced by a field-like STT generated by the current flowing through the MTJ. A field-like STT will tend to set the MTJ to anti-parallel direction with a low applied voltage. At high applied voltage, the field-like STT will switch its direction and tend to set the MTJ to parallel direction. Therefore, the MTJ can be selectively switched in either direction with a voltage applied in a unipolar direction depending on the pulse amplitude.